NH4+ vs. H2O

>> Imagine you are interpreting an electron density map for a
> new protein crystallized from ammonium sulfate. And you're
> at the stage of adding water molecules. How do you decide
> whether you have a water, or an ammonium ion close to a
> carboxylate on the protein?
In my opinion, you don't, unless you can make some positive chemical
argument about what it is an NH4, rather than an H2O (such as 3 or 4
tetrahedrally-arranged hydrogen bond ACCEPTORS). You'll certainly
never be able to tell from the number of electrons -- unless the
resolution & quality of your maps is a _lot_ better than mine! (What
are you working on? Crambin?)
I would be interested to know what others think.
Adrian
Adrian Goldman | Internet: Goldman at Ala.BTK.UTu.Fi
Centre for Biotechnology, | Bitnet: AGoldman at Finabo
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